1. Field of the Invention
The invention relates to noise reduction and, more particularly, to an apparatus for reducing fringe interference of light created in an optical system of a laser spectroscopy system, where the apparatus comprises an electromagnetic actuator for generating, along the laser path, physical translational vibration of an optical element of the optical system and a control device for controlling the amplitude and frequency of the vibration.
2. Description of the Related Art
The dominating source of background noise in laser absorption spectrometry is from the creation of spurious standing optical waves in the optical system, i.e., etalon effects. Etalon effects are created by multiple reflections from various optical surfaces, such as mirrors or windows. The etalon effects manifest themselves in a detected signal as interference fringes that can easily obscure an analytical signal from a sample. Etalon effects are extremely difficult to eliminate, even if a high quality anti-reflection coating is used because the interference pattern is deterministic rather than random. Consequently, normal averaging of the laser scans fails to reduce the interferences. One well-known and effective way to reduce the fringes is to vary the path length of the stray components by vibrating the position of the optical component that contributes to the creation of an etalon signal.
U.S. Pat. No. 4,684,258 to Webster describes the insertion of a vibrating Brewster plate between two etalon creating surfaces and thus periodically changing the optical path length of the etalon. U.S. Pat. No. 4,934,816 to Silver, et al. discloses a similar mechanical approach, where etalon effects in a multi-pass cell are reduced by the introduction of a vibrating mirror. In both cases, the vibration frequency is asynchronous with the laser modulation frequency so that the fringe pattern due to etalon effects will be averaged out. Moreover, the Webster and Silver, et al. approaches both use a triangular waveform to drive the plate and mirror into oscillation, respectively.
A triangular waveform provides a greater level of etalon fringe reduction in comparison to square or sinus waveforms, because the time spent by the vibrating element at the turning points is minimized. Unfortunately, this approach has two drawbacks. Firstly, generation of a triangular waveform requires a highly linear electromechanical transducer and imposes high requirements on the electromechanical setup. Secondly, in practice, the vibration amplitude of the optical element must be more than 30 Free Spectral Ranges (FSRs) or 15 laser wavelengths to obtain a sufficient reduction of the etalon effect. This becomes especially impractical when longer laser wavelengths are used thus imposing higher power consumption and placing higher demands on the mechanical components. Existing commercial instruments, which typically use vibration transition harmonic wavelengths in the range of 0.76 to 1.5 μm, would then require vibration amplitudes from around 10 to 25 μm. However, optimal laser wavelengths for laser spectroscopy are between 3 to 20 μm, where strong absorption signals exist due to molecular vibration transitions. As a result, larger vibrations amplitudes for reducing the etalon interference signal efficiently would be required. Consequently, the vibration amplitude is increased from around 10 to 25 μm to around 45 to 300 μm. As standard piezo-transducers have limited length expansion capabilities, their use will become virtually impossible for the lasers with longer wavelengths.
EP 1 927 831 discloses varying the optical path length of the passive optical cavity with a Gaussian (normal) distribution, where the standard deviation is at least one-quarter of the wavelength of the light. Thus, compared to triangular modulation, which requires vibrational amplitudes over several laser wavelengths, an efficient etalon averaging is already obtained at amplitudes following a Gaussian distribution with a standard deviation slightly above one-quarter wavelength. Another advantage is that, due to the character of noise modulation, there is no need to amplitude and/or phase control the modulating waveform, thus allowing a simplified hardware design. On the other hand, the random (noise) modulation contains high frequency components that exert higher acceleration forces on the moving mechanics.
All the known approaches use an open loop piezoelectric actuator for vibrationally moving the optical component that contributes to the creation of etalon signals.